For television program reception via satellite, a known configuration is to use a reception antenna composed of a parabolic reflector and of a source, a frequency-conversion unit transforming the signals received by the horn into electrical signals in an intermediate band, and a decoder that transforms the electrical signals into data or signals that are usable by a user appliance.
As regards satellite transmission, the transmission frequency band may turn out to be wider than the intermediate frequency band. In addition, a known method is to make use of horizontally and vertically polarized waves in order to double the capacity of the frequency band. Conversion units effect the choice of polarization and of band selection (high band or low band) during the intermediate frequency transposition.
A problem arises when several decoders are connected to the same conversion unit. All the decoders must position themselves in the same band which then renders the simultaneous display of two programs placed in different bands incompatible. Furthermore, the arrangement of one antenna per decoder is costly and not very aesthetic.
A known solution, shown in FIG. 1, consists in using a single antenna 1 with a multiple-output conversion unit 100. Thus, each decoder 2 from the same home can be independently connected to the conversion unit 100 while, at the same time, selecting the desired band and polarization for the user appliance 3 with which it is associated. Multiple-output conversion units 100 are often units having 2 or 4 outputs as shown in FIG. 2 or 3.
Currently, satellite television operators would like to have a service they can offer for their clients who wish to have two or more decoders with a reduced price per unit from the second decoder onward. In order to avoid a decoder offered at a reduced price being used in another installation, the system is designed to have one main decoder in the installation and secondary decoders that will only operate if they are in the same installation as the main decoder. In order to verify the presence of the main decoder, an exchange of keys between the two decoders can be used. In order to avoid having to add an additional connection between the decoders, the coaxial cable that connects a decoder to the antenna can be used. However, that requires having a secondary decoder connected to the same antenna cable as the main decoder, which in turn requires having all the decoders positioned in the same transmission band. Indeed, it is not possible to make use of a multiple-output conversion unit 100, since, if the switching matrix 110 is positioned in different bands for two different outputs, then there is total isolation between the coaxial cables corresponding to two different decoders, as can be seen in FIGS. 2 and 3.
A similar problem is encountered when it is desired to use a common satellite antenna connected to two or more decoders and to use a return channel. A single decoder can use the return channel at the same time; it is therefore necessary to make the decoders communicate with each other in order to determine which one is using the return channel. The antenna cable can also be used for exchanging synchronization information.
The same problems also arise when two or more decoders are used jointly with one or more antennas. We are then faced with a distribution network that incorporates one or more distributor(s), switch(es) or other coupling device. The antenna cables of the decoders can then be completely independent from each other.